1. Field of the Invention
The invention relates to an image coding/decoding apparatus and, more particularly, to an apparatus for decoding image information which has been variable-length coded on a block unit basis of a plurality of pixels.
2. Related Background Art
In recent years, in the field of the digital transmission of a color image, a high efficiency coding technique of information progresses and a high compression is being realized.
In association with such an improvement, a good image can be transmitted and received through a transmission line even at a low data rate. On the contray, an influence which is exerted on the image by an error of one word on the transmission line also increases. It is, therefore, necessary to take a countermeasure for the code error on the transmission line by an error detection code, an error correction code, and the like.
Particularly, in case of using a transmission line in which a deterioration of a transmission quality is presumed as in a magnetic recording medium, a communication satellite, or the like, it is necessary to particularly pay an attention to the countermeasure for such a code error.
FIG. 5 is a block diagram showing a schematic construction of a conventional image transmitting and receiving system.
In the diagram, reference numeral 101 denotes a terminal to which an image signal is supplied. The image signal supplied from the terminal 101 is converted into the digital signal by an analog/digital (A/D) converter 102. The digital image signal is coded by a high efficient coding circuit 103 and its information amount (band) is compressed.
The image information compressed by the coding circuit 103 is supplied to an error correction coding circuit 104 and a parity check bit to correct a code error is added to the image information (the image information is error correction coded). After that, the image information is sent to a transmission line 105.
On the receiving side, a data train transmitted via the transmission line 105 is once accumulated in a memory 106. The code error correction using the above parity check bit is performed in an error correction unit 107 which accesses to the memory 106. The image information which has been code error corrected is generated from the memory 106 and sent to a high efficiency decoding circuit 108. The decoding circuit 108 executes the decoding process opposite to the coding process by the high efficiency coding circuit 103, that is, expands the information amount (band) and returns to the original digital image signal. the digital image signal is converted into the analog signal by a digital/analog (D/A) converter 109. The analog signal is generated as an analog image signal from a terminal 110.
In FIG. 5, as a construction of the high efficiency coding circuit 103, namely, an image compressing method, various kinds of methods have been proposed. As a typical one of the color image coding methods, what is called an ADCT (Adaptive Discrete Cosine Transform) method has been proposed. The ADCT method has been described in detail in Takahiro Saito, et al., "A Coding Method of Still Image", the magazines of The Institute of Television Engineers of Japan, Vol. 44, No. 2, 1990, Hiroshi Ochi, et al. "The International Standard Tendency of Still Image Coding", the papers of the National Convention of The Institute of Image Electronics Engineers of Japan, No. 14, 1988, and the like.
FIG. 6 is a block diagram schematically showing a construction of a high efficiency coding circuit of an image using the above ADCT method.
In the diagram, it is assumed that an image signal which is supplied to a terminal 111 is a digital data train which have been converted into eight bits, namely, 256 gradations/color by the A/D converter 102 in FIG. 6. The number of colors is set to three or four such as RGB, YUV, YPbPr, YMCK, or the like.
The input digital image signal is immediately subjected to a two-dimensional discrete cosine transformation (hereinafter, referred to as a DCT) by a DCT converter 112 on a subblock unit basis of (8.times.8) pixels.
The data of (8.times.8) words which has been DCT converted (hereinafter, such data is referred to as conversion coefficients) is quantized by a linear quantization circuit 113. The quantization step size differs every conversion coefficient. That is, it is assumed that the quantization step size for each conversion coefficient is set to a value which is obtained by multiplying (8.times.8) quantization matrix elements from a quantization matrix generating circuit 114 by 2.sup.S times by a multiplier 116.
The quantization matrix elements are determined in consideration of a fact that the visibility for quantization noises differs every conversion coefficient of the (8.times.8) words. An example of the quantization matrix elements are shown in Table 1.
TABLE 1 ______________________________________ Example of quantization matrix elements ______________________________________ 16 11 10 16 24 40 51 61 12 12 14 19 26 58 60 55 14 13 16 24 40 57 69 56 14 17 22 29 51 87 80 62 18 22 37 56 68 109 103 77 24 35 55 64 81 104 113 92 49 64 78 87 103 121 120 101 72 92 95 98 112 100 103 99 ______________________________________
On the other hand, although 2.sup.S data are obtained from a data generator 115, S denotes 0 or a positive or negative integer and is called a scaling factor. The picture quality and a generation data amount are controlled by the value of S.
The DC component in each of the quantized conversion coefficients, namely, the DC conversion coefficients (hereinafter, referred to as a DC component) in the matrix of (8.times.8) is supplied to a one-dimensional prediction differential circuit 117, by which a prediction error is derived. The prediction error is subsequently Huffman coded by a Huffman coding circuit 118. Practically speaking, a quantization output of the prediction error is divided into a plurality of groups. The identification (ID) number of the group to which the prediction error belongs is first Huffman coded. Subsequently, the information indicating to which value in the group the prediction error is equal is expressed by an equal length code.
The conversion coefficients other than the DC component, namely, the AC conversion coefficients (hereinafter, referred to as an AC component) are supplied to a zigzag scanning circuit 119 and are zigzag scanned by two-dimensional frequencies from a low frequency component to a high frequency component as shown in FIG. 7. The conversion coefficients in which the quantization output is not equal to 0 (hereinafter, such conversion coefficients are referred to as significant coefficients) and the number (run length) of conversion coefficients which exist between the significant coefficients just before those conversion coefficients and in which the quantization output is equal to 0 (hereinafter, such conversion coefficients are referred to as insignificant coefficients) are generated as a set from the zigzag scanning circuit 119 to a Huffman coding circuit 120.
In the Huffman coding circuit 120, the significant coefficients are classified into a plurality of groups in accordance with the values of the significant coefficients and the group ID number and the above run length are Huffman coded as a set. Subsequently, the information indicating to which value in the group the prediction error is equal is expressed by an equal length code.
Outputs from the Huffman coding circuits 118 and 120 are multiplexed by a multiplexing circuit 121. The multiplexed signal is supplied as a coded output from a terminal 122 to the error correction coding circuit 104 at the post stage.
According to the high efficiency coding as mentioned above, even when an information amount is compressed into a fraction of an integer, no image deterioration occurs and the information can be compressed at an extremely high efficiency.
However, when the information is compressed at a good compression efficiency as mentioned above, namely, at a high compression ratio, an influence which is exerted on the image by one code error becomes serious.
For example, in the case where the variable-length coding as mentioned above is executed, the subsequent decoding process cannot be performed, so that the image after the occurrence of the error is disturbed and it is very hard to see.
In recent years, particularly, such a kind of apparatus is used even for a transmission line such that there is also a case where the transmission quality changes by the weather and deteriorates as in the communication satellite, so that it is required to take a countermeasure to protect data for the occurrence of errors on the transmission line.